I guess he didn't have time to pack the rest of the costume so poor Kal-el figured "I guess they didn't wear anything on top of it." *puts on suit* Earthlings ask "Who is the guy in the underwear?" and Kal-el thinks "I'm proud to carry on my heritage. At least the Kryptonians would get me." Kryptonians visit earth, point/laugh "He is wearing his underwear!"